1. Field of the Invention
This invention relates to a wireless audio transmission system, and more particularly to a wireless audio transmission system that transmits audio data digitally via amplitude-keyed microwaves from a transmitter to a receiver.
2. Description of the Related Art
Currently available wireless headphone and loudspeaker systems generally transmit an analog audio signal via frequency-modulated electromagnetic waves or via infrared light. These systems usually have noticeable noise in the received audio signal, low dynamics and high distortion.
This invention transmits the audio signal digitally using electromagnetic waves, thus eliminating those disadvantages of previous systems.
U.S. Pat. No. 5,596,603 describes a system that transmits digital audio data via infrared light. Transmissions via infrared light require visual contact between the transmitter and the receiver and furthermore are prone to interference due to strong light, such as sun light. In U.S. Pat. No. 5,596,603, it is also suggested to transmit audio data using an HF carrier that is timed with the transmission data. Conventional transmission bands at VHF and UHF frequencies, however, don""t provide the bandwidth needed to transmit the audio data. Therefore, in the present invention the audio data is transmitted using microwaves where there are frequency bands that have the necessary bandwidth. It is possible to use present invention in conjunction with aforementioned U.S. Pat. No. 5,596,603.
There are several systems that allow wireless transmission of data using microwaves, such as Wireless LAN systems. Also, there are DAB (Digital Audio Broadcast) systems that transmit audio data to portable receivers.
In the case of wireless headphones, such a system has to be of low cost and the receiver has to be of small size and has to have very low current consumption. In addition, such a system has to maintain continuous data transmission without dropouts due to multipath fading.
Multipath fading occurs frequently in indoor environments due to the cancellation of several signal components that are reflected and received by the receiver. This is a major problem in microwave transmission systems due to the short wavelength of the electromagnetic waves which are very well reflected by walls and obstacles.
To solve the problem of multipath fading, there is a variety of known diversity reception systems. One well-known method is the presence of multiple antennae (antenna diversity). Based on various criteria, one of these antennae is selected, thus switching from one antenna to another. Other systems incorporate adjustable phase shifters in order to achieve a constructive addition of these antenna signals or allow to switch the amplification/attenuation of one antenna as described in DE Pat. No. 4,310,256 A1.
Systems that implement switching, however, have a very high bit error rate while they are switching. They are mostly designed for packet-oriented data transmissions but are not suitable for a continuous data transmission such is the case for audio data transmissions.
Also, these systems only switch when a certain lower threshold is reached and thus often receive from an antenna that has the worse signal. Known Wireless LAN systems usually are not capable of maintaining a continuous data transmission. DAB receivers, however, are rather complex systems due to the OFDM transmission technique employed and are not suitable for this application due to high cost, high space requirements and high current consumption.
Therefore, it would be desirable to have a digital wireless audio transmission system of low cost that includes a receiver of low space requirements and low current consumption and that is capable of maintaining a reliable continuous transmission.
It is, therefore, an object of the invention to provide a system capable of digitally transmitting audio in which both the transmitter and receiver are of low cost.
It is another object of the invention to provide a system that includes a receiver with low space requirements and a low current consumption.
Another object of the invention is to provide a system that is capable of maintaining continuous reliable transmission without dropouts due to multipath fading.
Using a direct conversion receiver as the radio-frequency receiver, provides a solution to the objectives of low cost of the system and low current consumption of the receiver since no local oscillator signal is generated. Also, there is no need for a mixer stage to downconvert the receiver radio-frequency signal or an IF filter or IF amplifier stage for an intermediate-frequency signal. In addition, there is no need for expensive RF shielding due to the fact that no local oscillator is needed which otherwise would be necessary in order to comply with local regulations regarding maximum spurious radiation.
To fulfill the objective of high robustness of the transmission and to minimize the effects of multipath fading, the radio-frequency signal is frequency modulated with a periodic spreading signal and then amplitude modulated with the serial digital data to be transmitted. According to the invention, the frequency of the periodic spreading signal is chosen to be higher than the bit rate of the serial digital data.
To further increase robustness of the transmission, the transmitted signal is received via multiple antennae by the radio-frequency receiver. According to the first aspect of the invention, the receiver radio-frequency signals from these antennae are added via an adder, and the sum signal is amplified and detected by a small-signal detector. It is further suggested to amplitude modulate at least one of the receiver radio signals by an antenna modulator signal that has the same frequency as the transmitter spreading signal and a fixed relative phase deviation.
According to the second aspect of the invention, multiple detection signals from separate radio-frequency receive branches are added and the sum signal is then low-pass filtered and amplified. The small-signal detectors of the individual receive branches are decoupled for radio frequencies.